The invention relates to electrochemical batteries of the type having half cells in operative electrochemical contact.
There is a societal need for cost effective batteries having a high energy density and a moderate to high discharge rate. Such batteries can be used for applications ranging from flashlights to electrically powered vehicles.
A variety of metal/molten sulfur batteries have been presented with a primary focus on sodium/sulfur cells. The light weight of sulfur makes these systems attractive for electrochemical energy storage; however material constraints associated with the requisite high temperatures, corrosion, thermal cycling, and cell fabrication have slowed their development. Typically, such cells are designed to operate at temperatures of 300.degree. to 350.degree. C., to maintain the sodium, sulfur and reaction products in a liquid state and to obtain adequate electrolyte conductivity. See, e.g., D. Linden, Handbook of Batteries, McGraw-Hill, NY (1984).
At low (room) temperatures, elemental sulfur is a solid, an insulator, and highly insoluble; elemental sulfur is not expected to provide a useful cathode material at low temperatures.
In 1989, a low temperature alternative sulfur battery was presented. See Licht, U.S. Pat. No. 4,828,942, which is hereby incorporated by reference. Licht '942 describes a water-based battery using electrolytes which by mass could accommodate a high concentration of reducible sulfur. Indeed such batteries may be able to achieve sulfur concentrations that provide more reducible sulfur (by mass) than water. The resultant electrolyte retains high coulombic capacities similar to those in molten sulfur batteries, yet operates at moderate temperature and is highly conductive. The maximum charge density is described for a cell containing excess K.sub.2 S.sub.4 (potassium tetrasulfide) in contact with a saturated K.sub.2 S.sub.4 solution, with a maximum storage capacity of 2.8.times.10.sup.6 C/kg (coulombs per kilogram), limited by the solid K.sub.2 S.sub.4 capacity. The room temperature sulfur electrolyte provides a conductive, reversible, and high capacity half-cell storage material for the battery.
Choices for the second half-cell to combine with the sulfur cell described above (Licht U.S. Pat. No. 4,828,942), such as tin and highly reduced sulfur, generally have led to low battery voltages (0.6 volt and less). See, e.g., Licht, J. Electrochem. Soc. 134:2137 (1987). There is a large energy difference between sulfur and various alternative choices for second half-cells to complete the battery. Although this large energy difference provides the theoretical possibility of high energy storage, it also can lead to rapid undesired chemical reaction between the materials in the battery.
An object of the invention is to provide a safe and reliable battery capable of producing high energy densities. Another object of the invention is to provide a battery made of relatively inexpensive materials.
Another object of the invention is to provide a battery that can include seawater in the solutions, so that the battery will operate when immersed in seawater or the like.